[ObjC][ARC] Honor noescape attribute for -Warc-retain-cycles

[arphaman]

Summary

This patch ensures that -Warc-retain-cycles doesn't warn about captures in blocks that are passed into parameters that have a noescape attribute.

[rjmccall]

lib/Sema/SemaChecking.cpp

11661

Failing to find a capturing expression will overwhelmingly be the controlling condition here, so I would advise switching the order of the checks.

Otherwise, this patch looks good.

lib/Sema/SemaChecking.cpp

   } else {
     assert(msg->getReceiverKind() == ObjCMessageExpr::SuperInstance);
     owner.Variable = getCurMethodDecl()->getSelfDecl();
     owner.Loc = msg->getSuperLoc();
     owner.Range = msg->getSuperLoc();
   }
 
   // Check whether the receiver is captured by any of the arguments.
-  for (unsigned i = 0, e = msg->getNumArgs(); i != e; ++i)
+  const ObjCMethodDecl *MD = msg->getMethodDecl();
+  for (unsigned i = 0, e = msg->getNumArgs(); i != e; ++i) {
+    // noescape blocks should not be retained by the method.
+    if (MD && MD->parameters()[i]->hasAttr<NoEscapeAttr>())
+      continue;
     if (Expr *capturer = findCapturingExpr(*this, msg->getArg(i), owner))
       return diagnoseRetainCycle(*this, capturer, owner);
+  }
 }
 
 /// Check a property assign to see if it's likely to cause a retain cycle.
 void Sema::checkRetainCycles(Expr *receiver, Expr *argument) {
   RetainCycleOwner owner;
   if (!findRetainCycleOwner(*this, receiver, owner))
     return;
 

test/SemaObjC/warn-retain-cycle.m

__block void(^myBlock)(void) = ^{
             myBlock();
         }
         else {
       myBlock = ((void*)0);
         }
    };
 
 }
+
+typedef void (^a_block_t)(void);
+
+@interface HonorNoEscape
+- (void)addStuffUsingBlock:(__attribute__((noescape)) a_block_t)block;
+@end
+
+void testNoEscape(HonorNoEscape *obj) {
+  [obj addStuffUsingBlock:^{
+    (void)obj; // ok.
+  }];
+}